1. Field of the Invention
The present invention relates to a novel erosion resistant, low signature liner developed for solid rocket motors, and rocket motors with such a liner.
2. Description of Related Art
Rocket motors employing solid propellants typically comprise a rigid outer casing or shell; a heat insulating layer (insulation) bonded to the inner surface of the casing; a liner layer bonded to the insulating layer; and a solid propellant grain bonded to the liner layer. The propellant grain is typically center perforated, and these center perforations generally contain fins or slots to tailor rocket motor ballistic performance. These fins and slots allow the flame front to reach localized regions of the liner and insulation prior to motor burn out which exposes these localized regions to high temperature and high velocity gas flow for an extended period of time. The insulation is generally fabricated from a composition capable of withstanding the high temperature and high velocity gases produced when the propellant grain burns, thus protecting the casing. However, the liner and insulation combustion products typically contribute significant amounts of smoke to the rocket motor exhaust plume.
The typical liner contains a polymeric binder, a bonding agent to reinforce the propellant (bond line) near the liner interface, and solid fillers to provide mechanical strength and rheology control for processing. The liner must be compatible with and bond to the propellant and insulation or case surface. For this reason the binder used in the liner typically, but not necessarily, has the same chemical functionality as the binder used in the propellant. The liner will therefore normally have the same curable polymer and curative as the propellant binder. The percent of binder used in the liner depends on requirements of the specific bond line system and can vary over wide ranges, such as 40% to 80%. The bonding agent used in typical liners is present in the uncured liner in an amount from 2% to 6%. Fillers that are normally used include asbestos, carbon powder, silicon dioxide, and sometimes titanium dioxide or other inorganic materials. The amount of filler typically ranges from 20% to 50%. Asbestos is undesired due to health hazards.
Advanced tactical rocket motors require certain stealth characteristics including a low observable exhaust plume in the IR and visible light spectra in order to avoid detection. This is typically achieved through the use of non-aluminized propellants or solid propellants containing low amounts of aluminum. However, combustion products from the liner and insulation can also contribute significantly to the exhaust plume signature. The use of both a liner and an insulator can add smoke to the exhaust plume and increase the cost and inert weight of the rocket motor. These traits are undesirable from the standpoint of economics, performance and a low signature plume.
Prior efforts to reduce the liner signature include the use of a titanium dioxide filler in the High-speed Anti Radiation Motor (HARM) liner. This liner has poor erosion resistance, however, and a substantial thickness of additional smoke generating insulation is required for protecting the rocket casing.
Flame retardants have been used commercially as visible smoke and afterglow suppressants for materials and coatings but have not been evaluated for IR signature, laser transmittance, radar cross section and other stealth qualities required for low signature rocket motor application.
Prior techniques to increase the erosion resistance of materials have included the use of asbestos, aramid and polybenzimidazole (PBI) fibers and fiber reinforced zinc borate (ZB) and alumina trihydrate (ATH) compositions, such as described in the literature, including U.S. Pat. Nos. 4,595,714; 4,656,095; 4,956,397; and 4,600,732. Except for asbestos, these fibers have typically not been used in liners due to processing problems attributable to the high viscosities due to the presence of fibers.
Despite these and other efforts, rocket motors with liners which have demonstrably improved stealth performance, and liners with better erosion resistance, reduced weight penalty, and more flexibility in fabrication have long been desired.
It has been desired to produce rockets with improved liners, and particularly solid propellant rockets equipped with such a liner, which have lower visibility exhaust signatures and avoid (or have at least reduced) high volumes of visible smoke. In particular, it would be a further advancement in the art to provide a liner which exhibits such low visible and IR smoke emissions as to be suitable for low signature, e.g. stealth, rocket motor applications.
It has also been desired to have liner formulations with erosion resistance at least about comparable to that obtained using conventional fiber-containing rocket motor insulation, but which are also capable of being successfully used as sprayable or pourable liner coatings. It has therefore been desired to avoid the high viscosities and processing problems heretofore associated with the high fiber loadings required to achieve a liner having good erosion resistance.
It would be an advancement in the art to provide a single layer of a liner material that would also bond the propellant grain to the casing, and that is heat and erosion resistant to protect the casing. This would reduce rocket motor fabrication costs and reduce inert motor weight to improve motor performance efficiencies.
It would be an advancement in the art to provide a liner that is erosion resistant and free of high fiber loadings.
It would be an advancement in the art to provide a liner formulation, e.g., curable composition, that has a sufficiently low viscosity for spray or brush application.